HEAT 



101 



-50" C. (Salt and Mail, 1943). Some de- 

 terminations made in the light of these con- 

 siderations by puncturing the insects with a 

 thermocouple are given in Table 6. 



For the great group of insects of the tem- 

 perate regions that do not survive under- 

 cooling and subsequent freezing, the larg- 

 est class of insects in temperate North 

 America, the undercooling temperatures ap- 

 pear to be more important than the freezing 

 temperatures. The other tvi'O classes of in- 



artificial dehydration, and others are not so 

 affected. Lepticoris may lose a fifth of its 

 weight by artificial drying without showing 

 any change in the critical supercooling 

 temperature. The converse is also true: an 

 increase in water content by natural means 

 may, or may not, alter the ability to under- 

 go supercooUng. 



Animals exposed annually to seasonal de- 

 creases in temperature typically show sea- 

 sonal variation in the location of their 



Table 6. Mean Undercooling and Corrected Freezing Temperatures Based 



on Ten Insects of Each Species (Data from Ditman, Voght, 



and Smith, 194S) 



Active 



Anasa tristis female adults 

 Anasa iristis male adults. . 

 Pyrmtsta nubilalis larvae . . 



sects with relation to cold hardiness are: 

 ( 1 ) those that are extremely cold hardy and 

 survive undercooling and freezing and are 

 killed only by long exposure to low tem- 

 peratiires or by one or more sudden changes 

 in temperature; and (2) the noncold-hardy 

 nonhibemating insects that succumb to low 

 temperatiires even without freezing. 



The development of ice crystals within 

 the body is much more harmful than is a 

 lower temperature when ice formation is 

 avoided as a result of supercooling. An in- 

 crease in the degree to which supercooling 

 occurs in connection with cold hardiness is 

 usually associated with dehydration, but 

 the whole set of relations is far from simple. 

 For example, full-grown larvae that are still 

 feeding, prepupae, and pupae of the moth 

 Ephestia all have nearly the same percent- 

 age of water, yet they show critical su- 

 percooling points of —5.8°, —8.0°, and 

 —21.3° C, respectively. Some animals may 

 have their critical temperature lowered by 



critical supercooling temperature. Beetle 

 larvae that bore in oak wood, for example, 

 showed a supercooling point of —22° and a 

 reported freezing point of —12,8° C. in 

 February. In July, similar larvae super- 

 cooled only to —2° and froze at —0.8° C. 

 Insects from stored grains that are not or- 

 dinarily exposed to temperature extremes do 

 not exhibit this seasonal periodicity, al- 

 though they show much variation in cold 

 hardiness. 



The physiological explanations of these 

 complex phenomena are obscure. Although 

 supercooling presents certain resemblances 

 to the physical supercooling of distilled 

 water, the phenomena associated with cold 

 hardiness are much more complicated. Wig- 

 glesworth (1939, p. 366) sums up the 

 physiological situation thus: 



" , , . Ice crystals forming in solutions rich 

 in hydrophyllic colloids are liable to become 

 covered with a sheath of dehydrated colloid 

 and thus fail to 'seed' the entire solution; the 



